1. Field of the Invention
The present invention relates to vapor collection for trace constituent analysis, and more particularly, to vapor sampling by means of adsorbent polymers in which gases or liquids containing vapors are passed through beds of particles of adsorbent materials for further desorption of the vapor constituents into a flow stream for subsequent analysis.
More particularly, the present invention is related to a valveless, low power, ambient temperature vapor sampling system in which an in-line "trap", i.e. sorbent tube, containing adsorbent material, mechanically opens directly to a surrounding gaseous or liquid media for sample introduction and then seals to a unit containing a gas chromatographic column for thermal desorption and vapor injection into an analyzing system.
2. Description of the Prior Art
Vapor sampling by means of adsorbent polymers is a standard method of vapor collection for trace constituents analysis. In this process, gases containing vapors are passed through beds of particles of adsorbent material. Common examples of adsorbent materials include: Tenax TA, graphitized carbon black, and XAD Amberlites. Since materials differ in the affinities and capacities for absorbing the vapors of different compounds, the choice of adsorbent is based upon the sampling requirements and the specific vapors sought for collection.
Once vapors are sampled and adsorbed onto the surface of the adsorbent materials, the vapors are typically relieved by heating the adsorbent in the presence of a gas stream flowing past the adsorbent. This reversal of the process is typically called "desorption".
For many sampling applications, adsorbents are packed into small tubes, typically about 1/4" or less in diameter, which are commonly called "sorbent tubes" or "traps". Since the sampling process can dilute vapors from a large volume of air onto a small amount of adsorbent and then desorb these vapors into a much smaller volume for subsequent analysis, the process is an important means for concentrating dilute vapors prior to analysis. For this reason, the process is sometimes called preconcentration and the sorbent tube or portion of the apparatus containing the sorbent tube is often referred to as a preconcentrator. The aforementioned process is an important part of many methodologies for analyzing vapor constituents in air.
The same process is routinely also applied to the analysis of volatile chemicals which are trace contaminants in the water using "purge and trap" analysis. A liquid sample is stripped of volatile constituents by bubbling a large volume of a purge gas, such as helium, through the liquid sample which then passes through a sorbent tube. The gas flow effectively transfers the volatile constituents from the liquid to the adsorbent surface. Heating the liquid sample can improve the effectiveness of the purging process. Following this transfer, the sorbent tube is heated to desorb the vapors into a flow stream for subsequent analysis.
One of the most common methods for sampling of airborne contaminants with sorbent tubes is to use a small pump to draw air through the sorbent tube. Small battery operated pumps are one of the options available for this purpose. Following sampling, the sorbent tubes are then loaded into an apparatus called a "thermal desorber", one of which is manufactured by Dynatherm Analytical Instruments, Inc., Kenton, Pa. Each sorbent tube is manually loaded and secured in a flow path using large thumbwheels with fittings having ferrules which slip over the end of the sorbent tube and are gas tight after compression with the thumbwheels.
For automated approaches using sorbent tubes, a sorbent tube is typically "plumbed" into a flow path controlled by a valve for switching between flows to be sampled and flows to carry away thermally desorbed vapors.
This approach has been used in small gas chromatographs for sampling and preconcentrating air samples for analysis. Two examples of chromatographs which use this approach, are the MINICAMS instrument manufactured by CMS Field Products Group, OI Corporation, Birmingham, Ala., and the Microfast GC manufactured by Analytical Specialists, Inc., Baton Rouge, La.
The operation of the preconcentrator tube in the MINICAMS instrument is described in U.S. Pat. No. #5,014,541, while the operation of the preconcentrator in the Microfast GC is described in U.S. Pat. No. #5,611,846. In both instruments, air is sampled through a check valve and onto an adsorbent trap. Additional flow paths with valving are then used to reverse the flow during thermal desorption to carry vapors away from the trap and into flow paths leading to chemical analysis.
FIGS. 1A, 1B, and 2 show standard in-line adsorbent trap preconcentrator. As shown in FIG. 1A, in the sampling mode, a vacuum pump (sample pump) and associated valve are activated drawing air through the check valve and the trap. During sampling, the flow restrictor 1 prevents pressurized gas in the analyzer detector from being significantly drawn through the trap in competition with flow through the inlet check valve pathway. In the injection mode, shown in FIG. 1B, the carrier gas valve 2 is activated to allow pressurized gas to reverse the flow through the trap, the flow restrictor 1, and on to the analyzer-detector. The zone 200 illustrates the portion of the apparatus that must be typically heated to prevent adsorption losses of semi-volatile compounds in the sampling pathways.
In a typical gas chromatograph system (GC) using a preconcentrator, shown in FIG. 2, the entire sample path must be heated including the inlet, the check valve, the four port compression feeding manifold, the plumbing line to the trap housing, the line leading to the vent for baking the trap of excess chemical vapors and the line leading to the flow restrictor 1. Due to the fact that these components are bulky, the power consumption requirement for "stand-by instrument ready" conditions is extremely high. The conventional heated inlet design, even when miniaturized, requires a warm-up time of approximately 30 minutes at an average power of more than 30W resulting in an energy requirement of approximately 16W-hours, while the subsequent GC analysis may only require an additional approximately 1-5W-hr over a few minutes time span by using high efficiency column-heating technology. Further difficulties experienced with standard preconcentrators as well as typical GC configurations using such preconcentrators, include:
1. The check valves typically have temperature limitations of approximately 200.degree. C., which limits the temperature range of the sampling line. This temperature criteria limits the sampling of semi-volatile compounds: i.e., compounds having ambient temperature vapor pressures of approximately 1 mm or less, as these compounds condense on the walls of the sampling paths if they are not sufficiently heated.
The check valve is also susceptible to particulate contamination which may cause valve failure since the valve leaks when not seated properly.
2. The sampling lines and sampling inlet to the instrument also need to be heated sufficiently in order to pass lower volatility compounds of analytical interest. To avoid condensation losses ("wall losses") with these compounds, the plumbing may require significant heating unless the flow paths including valves have a very large flow capacity.
In the '846 U.S. Patent discussed above, temperatures on the order of 200.degree. C. are typically required for the sampling of semi-volatile hydrocarbons. Heating these lines requires substantial power which may be a burden for small, battery operated instruments. A warm-up time for heating these lines can substantially increase the power burden to a small instrument by lengthening the time required for conducting a single or intermittent analysis. For example, a relatively large amount of power can be consumed through a 30-minute warm-up for an analysis only requiring 30-90 seconds of chromatography. Further, this approach results in continuously exposing heated surfaces at the inlet of an instrument which may also present fire and explosion safety concerns in industrial or hazardous environments.
3. The trap may be awkward to access, service and replace if it is integrated into the plumbing of the instrument. For example, if the trap becomes contaminated through excessive exposure to a difficult to desorb compound, it may be preferable to replace the trap rather than extensively bake the trap in an effort to slowly desorb the contamination. Additionally, the flexibility of introducing traps (or sorbent tubes) containing vapor samples to the instrument that exists with manual thermal desorbers is lost when they are plumbed in a somewhat more permanent plumbing arrangement.
Another disadvantage of the plumbed-in trap is that if the trap should mechanically fail for any reason (for example, one of the trap bed retainers has failed to contain the bed of adsorbent material in the trap), poor access to the trap can delay correct assessment of the instrument's performance problem.
4. Sample introduction with an in-line plumbedin trap is typically limited to gaseous samples while many samples of analytical interest are in liquid form. Liquid samples are not easily introduced quantitatively through the hot inlet and plumbing line and may contaminate the check valve and plumbing lines, as well as generate excessive solvent vapor for the trap and injection process to properly function.
A new system for vapor constituent analysis spare of disadvantages of the prior art systems is, therefore, desired for monitoring, sampling, and detecting trace compounds in the atmosphere, water, and other gases and liquids.